The present disclosure relates to a system for the support of tanks for liquids in vessels. More particularly the disclosure relates to a system for bearing or support of vertical forces on tanks for liquids in vessels at the base of the tanks and for horizontal forces at few places so that the bearing of forces that arise are transferred in an advantageous manner to the construction of the vessel.
It is advantageous that the capacities of a ship are exploited in the most efficient way while safety is maintained. The design and therefore the fastening of tanks thereto are influenced by the liquid that is to be transported. This is influenced by the environment that the liquid requires. Liquids that are to be transported may be, for example, foodstuffs that require cooling in order to maintain quality, while other liquids require an over- or under-pressure. An energy carrier such as a liquefied natural gas (LNG) transport vessel requires a tank having a storage temperature of around −160° C. at atmospheric pressure. For carbon dioxide (CO2) transportation, a requirement is, for staying fluent, a temperature of −60° C. in addition to a pressure of about 600 kPa. When transporting other liquids, other conditions apply. Together with dimensions and weight of such tanks, this lay the foundation for even minor improvements may result in large economic profit and competitive advantages.
In the art is known to use hardwood to transfer support forces from a tank to hull as pure pressure via hardwood layers or synthetic materials with similar properties. It is essential that the materials in these layers have very good temperature insulating properties and that they can endure the pressure that they are subjected to.
Hardwood is a suitable material for this purpose, but there are many synthetic alternatives. The products DEHONIT® and PERMALI, which are trademarks of the German company “Deutshe Holzveredlung Schmeing” are examples of products for such use.
Tanks intended for transport of liquids with boats are often formed as spheres, cylinders or prisms. U.S. Reissue Pat. No. RE 29424 describes supporting of tanks having a cylinder shaped cross section that rest on a skirt with opposite sides firmly fastened to a hull of a ship, claim 1. The '030 reissue patent describes another form of support, where a number of support units along a horizontal circumference of the tank is put into opposite positioned sleeves.
Vessels may also comprise high vertical cylinder shaped tanks. High tanks may be advantageous for transporting liquids because it will then have better opportunity to adapt the amount of liquid that may be transported with a given hull of a vessel. In addition there are of course other technical problems to addressed as a consequence of using such high tanks. An example of such consequences may be conditions regarding the stability of the vessel. Another and important example is the vertical and horizontal support of such high tanks.
International (PCT) Application Publication No. WO 2010/020431 describes a device for storing a self supporting vertical tank for LNG. It comprises a support arrangement that enables a horizontal relative motion between the tank and the foundation. In this way the tank may contract and expand according to the temperature of the tank without unwanted tension to appear.
The foregoing International Application also describes an arrangement with vertical support faces that are distributed evenly around the tank in two heights. In this way the tank is supported when horizontal forces are applied and pitching is prevented.
A disadvantage with self supporting big and high vertical tanks is that they result in large local load because it is not straightforward to distribute the load. Normally the load is either applied to a ring on the bottom of a ship or to the sidewalls of the hull of the ship. This is described in the '431 PCT publication.
There are mainly two different versions of cryogenic tank design for transport of LNG that are being used today. One is a self supporting tank while the other is a so called membrane type. The most common self supporting type is the Moss tank with a design owned by the Norwegian company Moss Maritime and is a spherical tank. One advantage with self supporting tanks like the Moss tank is that they are robust. One disadvantage is that they are not very efficient in that much space is wasted in the hull with spherical tanks.
The French company Gaz Transport & Technigaz (GTT) own some important designs of membrane type tanks. A membrane tank has a layer of corrugated metal that can maintain its proportions in a wide temperature range so that the tank can fill out the space inside a hull and thus rest on the inner bottom and on the walls of the hull. This results in a very efficient utilization of the space in a ship. Disadvantages of membrane tanks today are that they have a history of leakage and they are not as robust as self supporting tanks. Maintenance on membrane tanks therefore has to be done at frequent intervals and this adds to the cost of running such ships.